Various types of devices that transmit and receive moving images and video data have been proposed. For example, Patent Document 1 describes a thin client system that transmits and receives video data between a server device and a thin client terminal.
A moving image coding device in a thin client system generally employs a scheme of coding and transmitting image data of a rectangular region including an updated region in an image. For example, this type of transmission scheme is described in Patent Document 2. According to this transmission scheme, in a case that only a partial region in an image is updated, the need for coding and transmitting the whole image is eliminated and it is merely required to code and transmit only image data of the partial image, so that it is possible to reduce the amount of data to be transmitted.
In particular, regarding a video image displayed on a computer screen, it is rare that the whole screen is frequently redrawn, and it is general that pixel values do not change at all except in a partial region. Therefore, data reduction by such partial coding is quite effective.
On the other hand, as a coding scheme for more general moving image signals other than those of a computer screen, there are MPEG scheme defined by ISO/IEC and H.26x scheme defined by ITU.
These moving image coding schemes promote reduction of the transmission coding amount by motion compensation using a motion vector showing what position in a reference image a pixel block to be coded resembles.
Further, a moving image coding scheme defied as MPEG-4 AVC or H.264 employs a function (Multiple Reference Frame) of being capable of holding a plurality of frames before and after a coding target frame and selecting a reference image to be used for motion compensation from the frames. This type of motion compensation scheme is described in, for example, Patent Document 3.
Use of Multiple Reference Frame makes it possible to select and use a frame that is the most appropriate for estimation from the plurality of held frames when performing motion compensation. Accordingly, even when a subject in a video image is temporarily hidden by another object and then appears on a frame again, it is possible to perform motion estimation by using a frame in which the subject is not hidden as a reference image, with the result that the efficiency of coding becomes higher than when using only one reference image.
[Patent Document 1] Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-503862
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2008-085502
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-186897
Regarding a video image on a computer screen, as well as a general moving image, there is a case that the same image is repeatedly drawn. For example, a wallpaper image on the background of a computer screen is redrawn every time a window opened thereon is closed. However, an object within the screen is not updated as frequently as a general moving image and, for example, there is a case that the same window image remains displayed in tens or hundreds of frames. Therefore, the wallpaper image on the background hidden by the window is redrawn after tens or hundreds of frames. Accordingly, the configuration of using a frame immediately before a coding target frame as a reference frame or the Multiple Reference Frame scheme using some frames before and after a coding target frame as reference frames cannot address such a situation.